Marijuana is the most abused illicit drug in the United States with an estimated 17.4 million past-month users. There has been a rise in use by young people in the last 6 years with a diminished perception of the drug's risks being attributed to its rise, especially in light of the drug's legal status. In fact, a recent national poll shows that fo the first time a majority of Americans favor legalized marijuana. In the past two years, the states of Colorado, Washington, Alaska, Oregon, and the District of Columbia have legalized the adult use of recreational marijuana. This unprecedented step comes with no direct scientific data available to understand how these new recreational use laws will affect marijuana consumption and public health. This proposal aims to address the question of how the retail sales of recreational marijuana affect its consumption and usage trends within a community. While surveys are the most used tool in studying drug abuse, new methods have recently emerged that enable a more objective assessment of illicit drug consumption. The use of sewer-based drug epidemiology was first employed less than a decade ago but has been shown to be a valid and complementary technique with traditional drug use indicators. Raw influent to a wastewater treatment plant (WWTP) from a population with a known catchment area and specific geographic boundary can be used to analyze trends in drug use over time. We propose to measure 11-nor-9-Carboxy-9-tetrahydrocannabinol (THC-COOH), the sewer available metabolite of the main active ingredient in marijuana, 9-tetrahydocannabinol (THC) as well as also measure the anthropogenic population biomarker 5-hydroxyindoleacidic acid (5-HIAA). In Aim 1, we will develop and validate an analytical method to determine the concentration of THC-COOH and 5-HIAA in wastewater. In Aim 2, we will determine correlation between estimates of total population marijuana use from measured THC-COOH levels in wastewater and from the usage data obtained from surveys. In Aim 3, we will explore more specific usage trends with levels found in wastewater to determine temporal changes such as weekday vs. weekend use. In Aim 4, we will explore the contribution of the recreational retail market to the total marijuana market by comparing WA State records of the grams of THC sold within the WWTP's bounds to the total THC-COOH levels found in the wastewater. At the conclusion of these studies, we will provide a timely understanding of how the sales of adult recreational marijuana impact its use within a population. This data can be used by local, state, and national planners as they assess and consider Washington's recreational marijuana law, as well as provide baseline data for future longitudinal studies.